Combined fluid pressure-vacuum relief and overturn protection in valving for transportation cargo tanks, are known in the prior art. Applicant's U.S. Pat. No. 3,568,695 is an example of such a combined valving arrangement. However in most of such existing valving arrangements, rollover protection is generally not adequately effective until approximately 180.degree. rollover occurs, and thus contents from the tank may be lost if less than 180.degree. rollover occurs, and certainly if less than 90.degree. rollover occurs.
Moreover, in many of the prior art rollover arrangements, heavy metal balls or the like are utilized to shut off the port opening and prevent emission of fluid from the tank upon rollover. Such mechanisms are subject to becoming out-of-round and leaking after a period of use, and thus the rollover protection mechanism is not always satisfactorily effective.
Other patents relating to safety valving for tanks are shown in U.S. Pat. No. 2,529,329 dated Nov. 7, 1950; U.S. Pat. No. 2,684,683 dated July 27, 1954; U.S. Pat. No. 2,783,913 dated Mar. 5, 1957; U.S. Pat. No. 3,123,087 dated Mar. 3, 1964; U.S. Pat. No. 3,302,658 dated Feb. 7, 1967; U.S. Pat. No. 3,757,987 dated Sept. 11, 1973; U.S. Pat. No. 3,765,435 dated Oct. 16, 1973; U.S. Pat. No. 3,913,601 dated Oct. 21, 1975; U.S. Pat. No. 4,023,583 dated May 17, 1977; U.S. Pat. No. 4,050,471 dated Sept. 27, 1977; and U.S. Pat. No. 4,095,609 dated June 20, 1978.
In most prior art arrangements wherein spring loading of outbreathing vents is embodied in the valving arrangement, the spring loading is generally so negligible that it cannot adequately resist the liquid head of a partially overturned or fully overturned cargo tank, and thus spillage of the tank contents occurs.